1. Field of the Invention
The present invention relates to a dynamic microphone and more particularly to a dynamic microphone in which a flux density in a magnetic gap where a voice coil is disposed is increased to improve sensitivity.
2. Description of the Related Art
A dynamic microphone is arranged such that a voice coil attached to a diaphragm is disposed in a magnetic gap formed in a magnetic circuit so that vibration of the diaphragm generates electric current in the voice coil, and therefore called an electro-dynamic microphone as well.
FIG. 3 shows in section an example of a conventional dynamic microphone. Reference numeral 1 indicates a magnetic circuit unit. A cylindrical permanent magnet (magnet) 2 is provided in the center of this magnetic circuit unit 1. A disk-shaped polar piece 3 is arranged in contact with one magnetic pole (for example, north(N) pole) of this permanent magnet 2. Further, a disk-shaped tail yoke 4 is arranged in contact with the other pole (for example, south (S) pole) of the above-mentioned permanent magnet 2. This tail yoke 4 is attached to a cylindrically shaped yoke body 5 to be fitted therein.
A ring-shaped magnetic gap G is formed between a perimeter face of the above-mentioned polar piece 3 and an inner circumference face at the front end of the cylindrical yoke body 5, thus constituting the above-mentioned magnetic circuit unit 1.
Further, reference numeral 11 indicates a diaphragm unit, in which a diaphragm 12 is constituted by a center dome 12a and a sub-dome 12b that is formed integrally with and around the center dome 12a. A voice coil 13 is integrally fixed to the diaphragm 12 using an adhesive (for example) at the boundary between the center dome 12a and the sub-dome 12b at the back of the diaphragm 12.
Furthermore, a circumferential edge of the sub-dome 12b which constitutes the diaphragm 12 is attached to the front end of a cylinder member 21 fitted onto the outside of the above-mentioned yoke body 5. In this situation, the above-mentioned voice coil 13 is arranged in the above-mentioned magnetic gap G.
In addition, reference numeral 23 indicates a resonator arranged in front of the diaphragm unit 11 through the above-mentioned cylinder member 21. Further, reference numeral 25 indicates a cap member in which a through hole 25a is formed in the center. This cap member 25 within which an acoustic resister 27 is arranged is attached to and fitted into the back end of the cylindrically shaped yoke body 5.
Incidentally, sensitivity of the thus arranged dynamic microphone generally depends on flux density in the above-mentioned magnetic gap G, length of the above-mentioned voice coil 13, and velocity of the voice coil 13 when the diaphragm 12 receives a sound wave.
Of these, the flux density in the magnetic gap G can be increased by narrowing the gap width. There is, however, a limit to reduction of the width of the magnetic gap G naturally, since the voice coil 13 is disposed within the magnetic gap so as to vibrate.
Further, in view of restrictions on a gap volume in the magnetic gap G and output impedance, it is hard to increase the length of the voice coil 13. Thus, it is often designed to have an impedance of 600Ω or less.
Furthermore, since the velocity of the voice coil 13 depends on the design of the acoustic mechanical vibration of the microphone unit, it is undesirable to increase the velocity taking into consideration the general directional frequency response.
Thus, the present applicant previously proposed the arrangement shown in FIG. 4 as a further improvement in the sensitivity of the dynamicmicrophone . This is disclosed in Japanese Patent No. 4573576. It should be noted that, in FIG. 4, parts which function similarly to those illustrated in FIG. 3 above are denoted by the same reference signs. Accordingly, the description of these parts will not be repeated herein.
The improvement shown in FIG. 4 arises having regard to considerable flux leakage taking place in the magnetic gap G in the magnetic circuit unit 1. By controlling the flux leakage in the magnetic gap portion, flux density in the magnetic gap is increased and the sensitivity of the microphone is improved.
Thus, as shown in FIG. 4, in the position opposed to the above-mentioned polar piece 3 and in front of the resonator 23, a second permanent magnet 31 is disposed to face the above-mentioned permanent magnet 2 and polarized so that both the facing sides have the same pole.
In other words, assuming that the polar piece 3 side of the above-mentioned permanent magnet 2 is polarized to have north(N) pole, the polar piece 3 side of the second permanent magnet 31 is also polarized to have north(N) pole.
According to the above-described arrangement, a loop of magnetic flux produced by the second-permanent magnet 31 and directed from north (N) pole to south (S) pole runs very close to the above-mentioned magnetic gap G, and this loop of magnetic flux acts so that the magnetic flux produced by the permanent magnet 2 and going to leak at the above-mentioned magnetic gap G may be pushed back to the magnetic gap G side.
As a result, the density of the magnetic flux produced by the permanent magnet 2 can be increased at the magnetic gap G and the sensitivity of the microphone can be improved.
Incidentally, in this type of dynamic microphone or speaker, it is known that using a molded magnet (alnico) as a permanent magnet for that magnetic circuit provides better sound quality than using a sintered alloy (ferrite, samarium cobalt, neodymium).
As compared with the samarium cobalt magnet or the neodymium magnet, the alnico magnet provides low magnetic energy and a microphone using the alnico magnet provides low magnetic energy as compared with that using the neodymium magnet, so that sensitivity of the microphone is low.
Then, as shown in FIG. 4, the second permanent magnet 31 is disposed to face the permanent magnet 2 and polarized so that both the facing sides have the same pole. It is envisaged that, in order to improve sound quality, the alnico magnet is used as the permanent magnet 2 which provides a magnetic field to the voice coil and the neodymium magnet having a large amount of magnetic energy is used as the second permanent magnet 31 for suppressing magnetic leakage in the magnetic gap. However, in the case of combining the above-mentioned magnets, a problem arises in that the permanent magnet 2 made of alnico is likely to be demagnetized by the second permanent magnet 31 made of neodymium.
Furthermore, according to the structure shown in FIG. 4, since it is arranged that the second permanent magnet 31 is disposed in front of the diaphragm, there is a limit to designing the above-mentioned resonator which is a cover member for improving high-frequency response of the dynamic microphone and a front sound terminal of the microphone. In other words, the dynamic microphone without having the second permanent magnet in front of the diaphragm can be designed to improve high-frequency response characteristics.